"does the fact that they terminated put them automatically into a reasonable business classification in that case?"
While I absolutely think that this is a reasonable business classification, I don't think it alters the fact that you have to decide, before you ever get to that stage, whether you think you can use the ABT for coverage or not in a plan where everyone is in their own group. If you believe you can, then I don't think it matters. If you believe that you can't (or the IRS position is that you can't) then again, I don't think it matters.
Interesting IRS comment, although I can't figure out what his real position is based on that! By the way, the sound you just heard was my head exploding.